Activity monitors have become popular tools for promoting exercise and a healthy lifestyle. An activity monitor may include a Photoplethysmography (PPG) sensor which provides data from which heart rate can be determined. PPG is an optical measurement technique in which light is directed towards the skin, and changes in blood volume pulsing under the skin are correlated to changes in reflected light. Accurate heart rate data is a desired goal. However, motion of the user during an activity like walking, running, swimming or biking can create motion artifact data in the same frequency range, e.g. beats per minute, as expected for the heart rate data from the sensor, thus adversely affecting heart rate data accuracy.